The present invention relates to an apparatus for manufacturing a light guiding plate used in a backlight device of a liquid crystal display device and its method.
The so-called edge light type light guiding plate, which lets in light from one side surface (light incident surface) of a transparent plate that is a square plate-like member, and which outputs the incident light from a surface (light output surface), is used in a backlight device of a liquid crystal display device for a word processor, personal computer, thin type television and the like. Such a backlight device is provided with a tubular light source placed on at least one side surface of the light guiding plate, and an element (hereinafter referred to as a “deflection element”) placed on a surface (light reflective surface) opposite to the light output surface that outputs light so as to change an angle of light that passes through the light guiding plate or an angle of light that is reflected by the light guiding plate.
Incident light from the light incident surface of the light guiding plate is changed in its direction at the light reflective surface and light output surface and the like, and outputs light from the light output surface. Moreover, light is totally reflected by the light reflective surface and transmits in the light guiding plate. Generally, the density distribution of deflection element and the shape thereof are determined such that luminance of light output from the light output surface becomes uniform at the entire surface of the light output surface.
The aforementioned deflection element can include the following three types {circle around (1)} to {circle around (3)}.
{circle around (1)} The surface of light guiding plate is coated with white ink that scatters or reflects light;
{circle around (2)} Dot pattern by which light is scattered or reflected is formed on the surface of light guiding plate; and
{circle around (3)} A light diffusion particle is dispersed in the light guiding plate.
The light guiding plate of type {circle around (1)} is manufactured by screen printing and the like. However, when the film thickness of print layer with white ink becomes uneven, distribution occurs in reflective power, so that luminance of light output from the light output surface becomes uneven. Further, if dust in the air is mixed into white ink during a print operation process and adhered onto the print side, light scattering is caused by dust, so that evenness of luminance cannot be obtained. Furthermore, the light guiding plate of type {circle around (3)} has difficulty in dispersing the light diffusion particle in a base material with high reproducibility in order to attain a predetermined density distribution.
For the above reason, type {circle around (2)} that is, the light guiding plate provided with the dot pattern (hereinafter referred to as “dots”), that is, the deflection element, which scatters and/or reflects light onto the surface, is used as a backlight device of a liquid crystal display device in many cases.
Conventionally, as a method for manufacturing the light guiding plate having dots, the manufacturing method with an injection moulding as disclosed in U.S. Pat. No. 5,776,636 is generally used. However, in the manufacturing method with the injection moulding, there is a problem existing in that the large-size injection moulding machine is needed in accordance with an enlargement of the light guiding plate resulting in rising in the manufacturing cost. Another problem exists in that a relatively long modeling time for pressuring and cooling was required to ensure high transcription rate.
As a method for solving the above problems, attention is focused on a method in which a resin substrate, made of acrylic, polycarbonate, polystyrene, and the like molded in the form of sheet beforehand, is subjected to surface treatment so as to manufacture a light guiding plate. As one example, Japanese Patent Laid-Open No. 11-147255 discloses a method in which the dots are formed onto the substrate of resin surface by a cylindrical roller having a pattern, corresponding to the dots, engraved on the surface so as to manufacture a light guiding plate.